Accelerometers are used for measuring vibrations in machines, such as gas turbines or jet engines. In aircraft engines there is a requirement for redundancy in almost all components, including monitoring equipment. It is therefore known to provide dual accelerometers, so that should one accelerometer fail, the other can be switched in to provide the necessary information to the engine control system.
One form of an accelerometer used in gas turbine engines comprises a piezoelectric crystal stack held on a pre-tensioned rod between a supporting base and a seismic mass. The sensor works by providing electrical signals indicative of the compression of the piezoelectric stack between the seismic mass and the supporting base.
A dual output accelerometer of this type is disclosed in U.S. Pat. No. 4,586,377. U.S. Pat. No. 4,586,377 describes a system in which two piezoelectric crystal stacks are located on a single pre-tensioned bolt, to provide two output channels. This system suffers from the problem that if one crystal stack breaks or crumbles, both signals are greatly reduced or lost. Also, if the pre-tensioned central rod breaks, both signals are lost.
Another problem with this type of accelerometer system is that when in an environment in which there are significant transverse vibrations, the output signals can be significantly distorted to the point where they are no longer useful. Furthermore, the pre-tensioned rod may fail. This is the case where the two accelerometers are placed on a single rod one above the other, or if they are placed side by side. If two accelerometers are placed side by side, when one accelerometer fails, it is quite likely that the other will also fail, as they are both subject to substantially the same conditions.
It is an object of the invention to provide a dual channel accelerometer that is more reliable under transverse vibrations. It is a particular object to provide an accelerometer that is more reliable under the conditions typically found in an aircraft gas turbine engine.